1. Field of the Invention
This invention relates to a liquid crystal projector, and more particularly to an optical system of a liquid crystal projector that is capable of minimizing the size and the number of optical elements of an optical system using three reflective liquid crystal displays.
2. Description of the Related Art
Nowadays, there has been highlighted a flat panel display that is capable of replacing a cathode ray tube display having a limit in the size of screen and a large system size and realizing both a thin thickness and a large-scale screen. The flat panel display includes a projector that projects a small-field picture onto a large screen on an expanded scale.
The projector employs a cathode ray tube or a liquid crystal display (LCD) as a display device for implementing a small-field picture. Recently, there has been mainly used the LCD to keep up with a trend toward a thin thickness of the projector. Generally, a liquid crystal projector adopts a transmissive or reflective LCD. The liquid crystal projector has been developed with intent to obtain small bulk, low weight and high brightness, whereas an LCD panel has been developed with intent to obtain high aperture ratio and high resolution. Thus, there has been used the reflective LCD panel so as to keep up with a recent trend toward a liquid crystal projector realizing high resolution, small bulk and low cost.
The liquid crystal projector takes advantage of a light emitting from a light source to implement a picture on the LCD panel. The liquid crystal projector images a picture of the LCD panel onto a screen using a projective optical system to observe a picture imaged on the screen. When the projector is implemented by directly projecting a picture of the LCD panel onto a rear screen, a projection distance should be assured between the screen and the projective optical system. Accordingly, since the rear side of the screen requires a relatively large space to enlarge a thickness of the projector, it is difficult to obtain a thin thickness of the projector.
In order to solve this problem, a reflecting mirror is introduced between the screen and the projective optical system to change a light path, thereby reducing a thickness of the projector. However, since an arrangement angle of the reflecting mirror has to be more than a critical angle thereof so as to project a picture onto the rear screen without any distortion, there is a limit in reducing a thickness of the system. Also, there is a limit in reducing a thickness of the system due to an inherent full length of an optical system consisting of an illumination system, the LCD and the projective lens system of the projector.
Referring to FIG. 1, there is shown an optical system of a conventional liquid crystal projector. The liquid crystal projector includes first and second fly eye lenses (FEL""s) 6 and 8, a polarizing beam splitter (PBS) array 10 and a first condensing lens 12 arranged between a light source 4 and a full-reflecting mirror 14, and a second condensing lens 16 arranged between the full-reflecting mirror 14 and a first Dichroic mirror 18.
A white color light emitting from a lamp of the light source 4 is reflected by a parabolic mirror to be progressed toward the first FEL 6. The first FEL 6 divides an incident light into the cell units and allows them to be focused onto each lens cell of the second FEL 8. The second FEL 8 converts an incident light into a parallel light for a specific portion and transmits the same to the PBS array 10. The PBS array 10 separates an incident light into linear polarized lights having any one light axis, that is, a P-polarized light and a S-polarized light. Then, a half wavelength plate (not shown) partially attached to the rear side of the PBS array 10 converts the transmitted P-polarized light into a S-polarized light. Thus, all incident lights are converted into S-polarized lights by means of the PBS array 10, thereby permitting most lights emitting from the light source 4 to be incident to picture implementing elements 26R, 26G and 26B of the LCD panel.
The first condensing lens 12 focuses an incident light from the PBS array 10 onto the full-reflecting mirror 14. The full-reflecting mirror 14 makes a full reflection of an incident light from the first condensing lens 12 and allows the same to be progressed toward the second condensing lens 16. The second condensing lens 16 focuses an incident light from the full-reflecting mirror 14 onto the first Dichroic mirror 18. The first Dichroic mirror 18 transmits a light at a blue color area in the incident lights while reflecting lights at a green color area and at a red color area having a larger wavelength than a blue light.
Further, the optical system of the liquid crystal projector shown in FIG. 1 includes a second Dichroic mirror 20, a first polarizing film 22R and a first polarizing beam splitter prism (PBSP) 24R arranged between the first Dichroic mirror 18 and the red LCD panel 26R, a second polarizing film 22G and a second PBSP 24G arranged between the second Dichroic mirror 20 and the green LCD panel 26G, a first relay lens 27, a second full-reflecting mirror 28, a second relay lens 29, a third polarizing film 22B and a third PBSP 24B arranged between the first Dichroic mirror 18 and the blue LCD panel 26B, a Dichroic prism 30 arranged among the first to third PBSP""s 24R, 24G and 24B, and a projection lens 32 installed in opposition to an light output surface of the Dichroic prism 30.
The second Dichroic mirror 20 reflects a light at the blue area in lights reflected from the first Dichroic mirror 18 and being incident thereto to progress it toward the second polarizing film 22G while transmitting a light a the red area to progress it toward the first polarizing film 22R. The second full-reflecting mirror 28 reflects a light at the blue area transmitted from the first Dichroic mirror 18 and being incident thereto to progress it toward the third polarizing film 22B.
The first and second relay lenses 27 and 29 are field lenses, which relay an imaging point of a light at the blue area to re-image the blue light onto the blue LCD panel 26B. Each of the first to third polarizing films 22R, 22G and 22B transmits only a S-polarized light parallel to its optical axis in the incident lights and allows it to be progressed toward the first to third PBSP""s 24R, 24G and 24B, respectively.
The first to third PBSP""s 24R, 24G and 24B reflects red, green and blue S-polarized lights transmitted from the first to third polarizing films 22R, 22G and 22B and being incident thereto and allows them to be progressed into the red, green and blue LCD panels 26R, 26G and 26B, respectively. Further, the first to third PBSP""s 24R, 24G and 24B obtain picture information from the red, green and blue LCD panel 26R, 26G and 26B to transmit red, green and blue lights converted into P-polarized lights, respectively and allows them to be progressed toward the Dichroic prism 30.
Each of the red, green and blue LCD panels 26R, 26G and 26B is a reflective LCD panel, which converts a S-polarized light reflected from each of the first to third PBSP 24R, 24G and 24B and being incident thereto into a P-polarized light to thereby implement a picture.
The Dichroic prism 30 obtains picture information from the red, green and blue LCD panels 26R, 26G and 26B to combine the incident red, green and blue lights and output the combined light to the projection lens 32. First and second polarization converting films (not shown) for converting P-polarized lights from the first and third PBSP""s 24R and 24B into S-polarized lights are arranged between the first and third PBSP""s 24R and 24B and the Dichroic prism 30, respectively. Accordingly, the Dichroic prism 30 reflects red and blue lights having a S-polarization component being inputted via the first and second polarization converting films into the projection lens 32. At the same time, the Dichroic prism 30 transmits a red light having a P-polarization component inputted via the second PBSP 24G into the projection lens 32, to thereby combine the red, green and blue lights. The projection lens 32 magnifies a picture being inputted from the Dichroic prism 30 and projects it onto the screen.
The conventional liquid crystal projector having as described above requires a plurality of Dichroic mirrors to separate a white light from the light source into red, green and blue colors because of an adoption of three LCD panels. Also, since the conventional liquid crystal projector uses a reflective LCD panel, it requires a plurality of PBSP""s differentiating paths of input and output lights on a basis of that reflective LCD panel. As an optical system of the conventional liquid crystal projector employing three reflective LCD panels requires a large number of optical elements, the size of the optical system is enlarged. Particularly, since the optical system has an inherent full length L, it has a limit in reducing a thickness thereof proportional to said inherent full length L. Thus, it becomes difficult to realize a thin thickness of the optical system.
Accordingly, there has been suggested an optical system that has a two-layer structure in which a color separator is arranged at the upper layer of the color-combining part and the projecting lens system so as to reduce a full length L of the optical system. However, the two-layer structure optical system also employs a large number of optical elements to have a limit in reducing its size.
Accordingly, it is an object of the present invention to provide an optical system of a liquid crystal projector that is capable of reducing the number of optical elements as well as the size of the optical system.
In order to achieve these and other objects of the invention, an optical system of a liquid crystal projector according to an embodiment of the present invention includes an illuminating unit for allowing a white light generated from a light source to have any one linear polarized light and a uniform light distribution; a color separator for separating the white light from the illuminating unit in accordance with a wavelength band to obtain a first color light and for polarization-converting the remaining lights in accordance with a wavelength band and then separating them into second and third color lights depending on their polarization components; first to third liquid crystal display panels using the first to third color lights from the color separator to implement first to third color pictures, respectively; a picture combining unit for combining the first to third color lights having acquired picture information from the first to third liquid crystal display panels; and a projection lens unit for projecting the combined picture from the picture combining unit onto a screen on an expanded scale.
In the optical system, the color separator includes a first Dichroic mirror for separating the white light in accordance with a wavelength band and allowing the separated first color light to be progressed toward the first liquid crystal display panel; and a first color selector for polarization-converting the remaining lights from the first Dichroic mirror in accordance with a wavelength band to differentiate polarization components of the second and third color lights. The optical system further includes a first polarized light separating prism for separating the second and third color lights from the first color selector in accordance with a polarization direction and allowing them to be progressed toward the second and third liquid crystal display panels, respectively.
Particularly, each of said first to third liquid crystal display panels is a reflective liquid crystal display panel for converting and outputting a polarization component of an incident light depending on the picture information; and said first polarized light separating prism for allowing the second and third color lights polarization-converted by acquiring picture information from the second and third liquid crystal display panels to be progressed into the picture combining unit. The optical system further includes a second polarized light separating prism for allowing the first color light from the Dichroic mirror to be incident to the first liquid crystal display panel and allowing the first color light polarization-converted by acquiring picture information from the first liquid crystal display panel to be progressed toward the picture combining unit.
In the optical system, the picture combining unit further includes any one of a third polarized light separating prism and a Dichroic prism for acquiring picture information from the first to third liquid crystal display panels to combine the first to third color lights being incident thereto via the first and second polarized light separating prisms and output the combined light.
The picture combining unit further includes a second color selector for selectively converting polarization components of the second and third color lights outputted from the first polarizing prism in accordance with a wavelength band and allowing it to be progressed toward any one of the third polarized light separating prism and the Dichroic prism in the same linear polarized state.
In the optical system, the first and second color selectors make a polarization conversion of the red light while transmitting the light at other wavelength band as it is.
The optical system further includes a half wavelength plate for converting a linear polarized light of the first color light having acquired the picture information from the second polarizing prism and allowing it to be progressed toward any one of the third polarized light separating prism and the Dichroic prism.
The optical system further includes a third color selector for polarization-converting the combined picture from the picture combining unit in accordance with a wavelength band and allowing it to be progressed toward the projection lens.
Herein, if a P-polarization screen for transmitting only a P-polarized light is applied to said screen, then the third color selector allows all lights going between the picture combining unit and the projection lens unit to be converted into P-polarized lights.
In example of another system, the picture combining unit includes a second Dichroic mirror for selectively reflecting and transmitting the first to third color lights from the first and second polarized light separating prisms in accordance with a wavelength band to combine a picture.
The optical system further includes first and second color light filters arranged between the second and third liquid crystal display panels and the first polarized light separating prism, respectively to enhance color purity.